This invention relates to reducing the power consumption of a microprocessor.
Microprocessors used in computers and other types of computing devices are typically power-sensitive components. That is, due to the large number of operations that a microprocessor performs, it is desirable to keep its power consumption per operation as low as possible, in order to reduce the energy usage of the device incorporating the microprocessor. In a computer, operation codes (opcodes) operate on registers, values in memory, values stored on the stack, the I/O ports, the bus, and so on. The opcodes are used to perform arithmetic operations and to move and change values. The opcodes operate on operands. Microprocessors convert the opcodes into sets of binary bits and perform operations using these sets of bits. Each bit in a set can have an on-state (typically represented as ‘1’) or an off-state (typically represented as ‘0’). Eight bits are usually referred to as a byte on most conventional processors, and two bytes are usually referred to as a word.
The basic set of commands, or instructions, that a microprocessor understands is termed a command set, or an instruction set. Various types of microprocessors can have differently sized instruction sets, and this difference often constitutes one of the main differences between the microprocessors. For example, RISC (Reduced Instruction Set Computer) microprocessors have relatively small instruction sets, whereas CISC (Complex Instruction Set Computers) processors have relatively large instruction sets.
When designing an Instruction Set Architecture (ISA) for a microprocessor, little thought is typically given to how the respective instructions in the opcode that is used in the microprocessor are assigned to bit sets. The number of bits in an opcode instruction typically depends on the total number of instructions that can be carried out by the microprocessor, and which bits are assigned ‘0’s and ‘1’s, respectively, is usually arbitrarily determined. In most microprocessors, 6-8 bits correspond to an opcode instruction. There is always a one to one correspondence between a particular opcode instruction and a set of bits forming a corresponding machine code instruction.
One of the most common operations of a microprocessor is to fetch instructions from a memory external to the microprocessor for execution by the microprocessor. This is a power intensive operation, as the number of signal transitions is high. Thus, in order to reduce the power consumption of the microprocessor, it would be desirable to construct the ISA in such a way that the number of signal transitions in its most common operations is as low as possible.